The present invention relates to an improved drill bit construction and step feeding method and, more particularly, to a drill bit, including current straight-type and under-cuttype drills, whose geometry and construction, such as flute/land ratio tapering, decrease positional error and tip deflection and improve hole quality. The step feeding method in accordance with the present invention includes controlling the size of each step in a specific relationship to prevent clogging of the drill flute.
In conventional high speed, high aspect ratio drilling of single or stacked printed circuit boards (PCBs) W as shown in FIGS. 17A-17D, the boards are held by a circular pressure pad 21, and air is suctioned through a hole or opening 23 in the pressure pad 21 to remove chips from the grooves of the drill bit 3 through a low pressure evacuation conduit 22. FIG. 17A shows the pressure pad 21 clamping the board or boards W with the drill bit 3 in the workpiece, while FIG. 17C is an enlarged portion of the drill bit on the board(s) showing the flow of air and the chip in the flute. FIG. 17B shows the drill bit 3 removed from the board(s) W and the pressure pad 21 above the board(s) in noncontacting relationship, while FIG. 17D is an enlarged portion of the drill tip showing clogging of the flute. With this "vacuum cleaner" method, the differential pressure cannot exceed one atmosphere; consequently, air flow velocity is not sufficient to clean badly clogged drill bits, and hole roughness and resin smear usually occur as a result.
As shown in FIGS. 18A, 18B, when the aspect ratio (i.e. depth/hole diameter) is eight or more, and not all the chips are removed from the bit as is the case in FIGS. 17A-17D, the drill thrust load increases with hole depth. The resultant drill bending results in hole position deviation at the bottom of the board stack, hole enlargement. Furthermore, hole smear can occur due to heating of the drill.
A step feed method of the general type shown in FIGS. 14A, 14B has made high aspect ratio drilling possible by greatly reducing or eliminating drill clogging. As is more fully described in U.S. Pat. No. 4,872,787, the depth of each drilling step (where each step is designated by a circled numeral) is selected such that serious drill clogging does not occur. After each thrust 1-3, 5 and 7 the drill is withdrawn, thereby removing the chips and cleaning the drill bit flute. Step feeding is repeated so that drilling of the board(s) occurs a little deeper each time occurs.
Conventional high aspect drill bits are illustrated in FIGS. 19 to 21. A typical drill diameter is 0.016", the web thickness is approximately 15% of the drill diameter, the flute/land ratio is approximately 2.0, the web taper is 1.5 to 2.0 per 100, the flute angle is 30.degree. to 35.degree., the material is K30 cemented carbide. We have found that the body length and flute length are either too long or too short for drilling printed circuit boards (PCBs). Chip clogging is a problem as shown in FIGS. 17A-17B, and to avoid this problem in some drills, the relief N.sub.2 shown in FIG. 21 is doubled to 0.002" from the relief N.sub.1, of FIG. 20. This expedient greatly increases, however, drill breakage and degrades positional accuracy and hole quality. Moreover, there is undesirable stress concentration at the flute end which constitutes also the body or shank end.
When such a drill bit is used for step feed drilling of the type shown in FIGS. 14A and 14B, the second moment of cross sectional area shown by the sinuous curve S.sub.1 in FIG. 19 demonstrates that there is very little difference between the cross section at the drill tip and the cross section at the shank end, so that resonance-related vibration, spindle runout, errors in centering the drill point, roughness on upper and lower board surfaces, resistance caused by glass filament bundles, and clogging of the drill flute can cause a radial load and a force in the direction of the arrow resulting in drill bending or even breakage. The second moment of area varies sinuously along the length of the drill at the positions shown at B going through several minimums which are essentially constant along the length of the drill bit. The amount of deflection, y.sub.1, is inversely proportional to the number and size of these minimums. In other words, the smaller the sizes are in magnitude, the larger the deflection.
Furthermore, the Young's modulus of K30 material is low. If the elastic limit is exceeded, permanent deformation results. When drilling a stack of three boards, each board being about 0.063" thick, the deviation of the hole at the bottom surface of the lower board due to drill bending can be large, thereby resulting in holes not centered in conductor lands and in unacceptably high board rejects. In addition, when the step feed method is used, damage to the edge of the hole will occur each time the drill is withdrawn and reinserted if the drill is permanently deformed.
FIG. 7 shows the drill tip deflection due to bending when a bending force is applied to the tip of different types of bit, and FIG. 8 shows the change in hole positioning error with the number of holes drilled, in this case the hole error at the rear of the stack of three PCBs. Curves D and E in each figure represent deflection and error for, respectively, conventional drill bits, and it can be seen that they are unacceptably high. Conventional drill bits use a constant or almost parallel taper as shown in Japanese LaidOpen Publication No. 61-50706 where the lead angle is in excess of 26.degree., the depth of the flute start is 70 to 80% of the drill bit radius and the flute end depth is 50 to 80% of the drill bit radius.
Another form of conventional drill bit employs a solid material with a flute/land ratio at the flute end of more than one (1) whereas the web taper range is from 0.1/100 to 5/100 as described in Japanese Laid-Open Publication No. 59-156719.
A known drill bit utilizing a flute cross-sectional radius which becomes larger toward the flute end of the bit is also described in Japanese Laid-Open Publication No. 61-226209. The flute angle remains constant throughout in this device.
Another known type of drill is shown in Japanese Laid-Open Publication No. 60-61110 which is characterized by a lead angle which becomes smaller toward the flute end toward the flute start of the shank. This configuration does not disclose a taper or the concept of the lead angle of the flute start being greater than the lead angle of the flute end.
There is nothing in the foregoing types of drill bits which overcomes the problems and disadvantages encountered in high speed, high aspect ratio drilling in terms of hole smearing, tip deflection and hole positional error.